The process of wet oxidation has been used for the treatment of aqueous streams for over thirty years. It involves the addition of an oxidizing agent, typically air or oxygen, to an aqueous stream at elevated temperatures and pressures, with the resultant "combustion" of oxidizable materials directly within the aqueous phase. The wet oxidation process is characterized by operating pressures of 30 to 250 bar (440 to 3630 psia) and operating temperatures of 150.degree. to 370.degree. C. Reaction is primarily carried out in the liquid phase since gas phase oxidation is quite slow. Thus, the reactor operating pressure is typically maintained at or above the saturated water vapor pressure, so that at least part of the water is present in liquid form.
Wet oxidation is applicable to streams with a chemical oxygen demand (COD) as low as 20 grams/liter (Perkow, H., R. Steiner and H. Vollmuller, "Wet Air Oxidation - A Review", German Chemical Engineering, 1981, 4, 193-201). Below this level energy inputs to the process are excessive, and other technologies such as biological treatment or carbon adsorption become more economical The advantages of wet oxidation over biological treatment and adsorption are reduced time and space requirements for treatment, destruction of chemicals toxic to microorganisms, destruction of non-biodegradable chemicals, and potential recovery or inorganic materials The upper limit of wet oxidation applicability is set by the temperature rise due to reaction, and is about 200 grams/liter of utilized chemical oxygen demand (COD). For wastes with higher heating values, incineration becomes a more attractive disposal alternative. When applicable, advantages of wet oxidation over conventional combustion include energy efficiency (evaporation of the process water is not required) and easier handling of inorganic constituents. Typically, the amount of oxidant required by the COD of the waste exceeds the solubility limit of oxygen or air, so that both gaseous and liquid phases are present in the reactor. Because oxidation is carried out primarily in the liquid phase, some provision for mixing must be made to facilitate transfer of oxygen to the liquid phase. Bubble columns, baffles, packed beds and stirrers have been used to achieve this goal.
The largest single application of wet oxidation is for the conditioning of municipal sludge. The COD reduction in this process is only 5 to 15%, the primary objective being sterilization and disruption of the organic matrix to improve the dewatering properties of the sludge. Following wet oxidation, the sludge is used for soil improvement or landfill, or is incinerated. Other uses of wet economical. are for the treatment of night soil, pulp and paper mill effluents, regeneration of activated materials. carbon, and treatment of chemical plant effluents. In these applications, COD removal is typically 90% or less.
Wet oxidation is limited not only in the degree of oxidation achievable, but also by its inability to handle refractory compounds. Because of the low temperatures relative to those found in normal combustion, reaction times are on the order of an hour, rather than seconds. Even with these extended reaction times many refractory organics are poorly oxidized. One means for improving the low temperature oxidation has been the usage of homogeneous or heterogeneous catalysts in the liquid stream. The process is significantly complicated by this approach because of catalyst deactivation, attrition, and recovery The low temperatures of the wet oxidation process also limit its usefulness for power recovery.
Barton, et al. (U.S. Pat. 2,944,396) in 1960 proposed adding a second stage of oxidation to wet oxidation processes, in order to overcome some of these drawbacks. In this patent, the unoxidized volatile combustibles which accumulate in the vapor phase of a wet oxidation reactor are conducted to a second reactor in order to complete their oxidation. In contrast to conventional wet oxidation conditions, the temperature in this second reactor is allowed to exceed the critical temperature of water of 374.degree. C. It thus becomes possible to produce a high enthalpy stream suitable for power generation, as well as to oxidize certain volatile compounds, such as acetic acid, which are refractory under normal wet oxidation conditions. Catalysts are frequently used in this process. While the process may be advantageous in the context of a wet oxidation plant, as a stand-alone process the Barton invention is of limited utility because its usefulness is constrained to clean, volatile fuels Such materials are too expensive for power generation on a large scale, and when occurring as a waste stream, are more economically treated by other means. The limited usefulness of the Barton process appears to be indicated by the lack of literature reports on commercial application.
In 1981, Dickinson (U.S. Pat. No. 4,292,953) proposed a modified wet oxidation process for power generation from coal and other slurry fuels in which, as heat is liberated by combustion, the entire reaction mixture exceeds the critical temperature (374.degree. C.) of water. The higher temperature would result in accelerated reaction rates, allowing shorter residence times and smaller reactors as compared to conventional wet oxidation. Dickinson's process teaches a process range of between 1000 and 10,000 psi, which covers the range both above and below the critical pressure of water (3206 psi or 221 bar). In the Dickinson process, the reaction commences at subcritical temperatures so that, similar to wet oxidation, the bulk of the oxidizing gas is found in a separate phase above the aqueous slurry. This feature results in a slow reaction initiation and a requirement for prolonged residence time in the reactor, though some improvement over conventional wet oxidation would be expected. To bring the reaction initiation to a higher rate, Dickinson proposed the use of alkali catalyst.
In U.S Pat. No. 4,388,199, Modell introduced the concept which is referred to as supercritical water oxidation. This process requires the use of supercritical pressures and temperatures, and oxidation is initiated at supercritical conditions. As a result, virtually the entire reaction occurs at an unexpectedly high rate. The use of liquid, solid or gaseous fuels is contemplated in this invention. The process has been successfully applied to a wide range of liquid organics on an experimental and pilot scale, with virtually complete destruction in less than a minute of residence time. Destruction efficiencies of 99.9999% are readily attained, making the process useful for the destruction of toxic and hazardous combustible wastes. In the supercritical water oxidation process, as compared to wet oxidation, a higher temperature rise due to reaction is tolerable, allowing the use of feeds with 300 grams/liter or more of COD. This is equivalent to a maximum feed fuel value of about 4650 kJ/kg (2000 Btu/lb.). Use of feed blending or regenerative heat exchange allows the treatment of streams with a wide range of fuel values. Alkali catalyst is not required in the supercritical water oxidation process.
In U.S. Pat. No. 4,380,960, Dickinson added the feature of high temperature initiation to the process. In this patent, several means of preheating the slurry feed are taught, and examples are given of embodiments in which the slurry is brought to supercritical temperature (and optionally supercritical pressure) at the inlet of the reactor. To allow reaction initiation in a vapor or supercritical fluid phase, rather than in a dense liquid phase as with wet oxidation, Dickinson taught the need for an alkali catalyst.
There are a number of patents for wet oxidation type processes carried out with a deep well reactor configuration. An advantage of this mode operation is that some of the requisite high pressure is generated by a column of fluid, allowing the use of relatively low pressure pumps and compressors. Representative of conventional wet oxidation, i.e., subcritical temperature and pressure, are Bauer (U.S. Pat. No. 3,449,247), McGrew (U.S. Pat. No. 4,272,383) and Kaufmann (U.S. Pat. No. 4,774,006). Kaufmann also extends the patent teachings to supercritical conditions. There are several other patents disclosing the use of supercritical water for oxidation with a deep well reactor configuration, for example, Burleson (U.S. Pat. No. 4,564,458), Titmas (U.S. Pat. No. 4,594,164) and Titmas (U.S. Pat. No. 4,792,408).
In addition to higher temperatures and hence reaction rates, much of the benefit of extending wet oxidation to supercritical conditions derives from the phase behavior in this region. Oxygen and many gaseous, liquid and solid combustibles are completely miscible with supercritical water, allowing intimate mixing of reactants in the aqueous phase. Mass transfer resistances due to the bulk separation of oxidant and fuel are not a concern, and open tubular or vessel reactors may be used. It is also found that, in the supercritical region, solubility of many inorganics is quite low. An important example is sodium chloride, which has a solubility as low as 150 mg/kg above 450.degree. C. at 250 bar. Thus, it is possible to purify the high temperature, high pressure aqueous stream of many inorganic materials normally considered highly water soluble by using an appropriate solids separation scheme. This fact is important because it is conceivable that upon removing the inorganics, the supercritical water stream may be directly used for power generation. Such a power cycle has an inherent advantage over conventional stream power cycles in that the heat of combustion need not be transferred across boiler tube walls to reach the working fluid. The mode of operation is analogous to a gas turbine, in which the combustion medium is also the working fluid. Unlike a gas turbine, however, dirty or wet fuels, such as coal or wood refuse, may be used. It is also to be noted that the operating pressure and temperature of the supercritical water oxidation process are similar to and compatible with current-day supercritical steam (water) power cycles. Plants with supercritical steam power cycles have been in operation for several decades.
Compared to wet oxidation, the greatly reduced residence time possible with supercritical water oxidation allows the use of considerably smaller reactors. Furthermore, the higher operating temperature permits more efficient recovery of the heat of reaction and potentially simplifies the schemes for solids removal. On the other hand, the higher temperature and pressure of operation require the use of more exotic construction materials, heavier schedule pipe, and greater compression and pumping costs. The fact that supercritical water oxidation operates in a region where corrosion phenomena have not fully been investigated further adds to the burden of materials selection. While the bulk of inorganic solids are removed via precipitation, the process stream may still contain a certain amount of dissolved solids, and the direct feeding of this stream to a turbine will require some development effort. In addition, the art of supercritical pressure steam power generation is less highly developed than that of subcritical pressure steam power generation. Consequently, there is still a need in the industry for a process that has the advantages of supercritical water oxidation without the disadvantages associated with supercritical pressure.